L0301P42 - Structure and Function of Bone
Bone Tissue *main collagen type if collagen type I *matrix is mineralised by calcium phosphate as “hydroxyapatite” crystals *function: **storage for calcium and phosphate **support for locomotion, protection of soft tissues (but may also cause harm if bone is not in the correct position) **maintaining haematopoietic tissues Compact Bone *organised into sheets (lamellar bone) *may also be concentric lamellae, forming osteonal bone **cylindrically arranged into osteons **also known as Haversian bone *interstitial lamellae **fragments of osteons **fill gaps between full osteons *circumferential lamellae **the wall of a long bone typically has inner and outer circumferential lamellae, with an osteonal core **fibre orientation of different layers are perpendicular to each other *fibrous tissue covering **periosteum - outside **endosteum - inside **carries numerous nutrient arteries to supply the bone *most blood moves through Volkmann’s canals (vessels run through them) Spongy (cancellous) Bone *similar to lamellar bone (and often continuous with) *the “sheets” are converted into tubes, known as trabeculae **very thick trabeculae may contain osteons *trabeculae are lined with endosteum *wholes in the trabeculae is filled with bone marrow *trabecular bone constantly suffers microfractures and is then repaired along  optimal lines of stress *this process ensures optimal force transmission and drives bone regeneration *the long term result is “younger”, “healthier”  bone (irrespective of chronologic age) Osteon Cells in the Osteon Osteoblasts *building bone *secrete collagen and ground substance *secrete matrix vesicles containing alkaline phosphatase *their release initiates mineralisation of the extracellular matrix *highly responsive to mechanical stimuli **bone is built along lines of force **loading a bone therefore initiates osteoblast activity Osteocyte *“dumb” osteoblast *old osteoblasts, surrounded by the extracellular matrix they produced *long processes remain in canaliculi **chain of communication between osteocytes by extending cytoplasm to reach each other, relaying cell activity information **when chain of communication is broken osteoclasts can clear the osteocytes **bone resorption and remodelling follows the pattern of these signals ***i.e.. old osteocytes communicate less, so are more likely to be absorbed, along with their surrounding matrix *can also resorb calcium from the matrix, send along canaliculi and into the blood supply **stimulated by increased levels of parathyroid hormone in the blood **the body’s major reservoir of calcium (bone) can therefore be accessed to increase blood calcium levels Osteoclast *phagocytic cell: clears bone and osteocytes *derived from bone marrow Howship’s Lacuna *ruffled border enables changes in the local environment *pH is lowered to facilitate decalcification *lysosomes can then be released to digest the organic components of the matrix *degradation of the matrix **under control of Parathyroid hormone **receptors on osteoblasts for PTH, calcitonin and other factors **PTH stimulates osteoblasts to send activating factors to osteoclasts *osteoclastic activity enhanced by high PTH **over stimulation causes demineralisation **leading to osteoporosis in late life (more in female than male) if adequate bone is not made in replacement (bone is broken down faster than it can be made) *calcitonin receptors on osteoclasts which can significantly decrease the osteoclastic activity (counteracts osteoblast signals) Osteoprogenitor Cells *precursors to osteoblasts/osteoclasts *derived from mesenchymal cells *form the innermost layer of cells lining bone **periosteum **endosteum (marrow cavities, osteonal canals, Volkmann’s canals) *when inactive = bone lining cell **function is not entirely known **may communicate with and support osteocytes and hence help direct patterns of bone remodelling Primary Woven Bone vs Mature Bone Left *primary woven bone *found when bone first forms or to repair a fracture *indiscriminate growth of bone where remodelling has not yet occurred Right *mature lamellar bone *osteons are formed in the longitudinal direction of the bone in response to force Ossification Endochondral Ossification *ossification from within cartilage *disruptions in the epiphyseal growth plate will result in issues with growth of the bone Intramembranous Ossification *primary centre of ossification forms in the centre of the mesenchymal tissue *cells differentiate into osteoblasts making woven bone matrix *in subsequent weeks, mineral into bone *occurs in mandible, clavicle and flat bones of the skull Epiphyseal Plate *supplies new chondrocytes from reserve chondrogenic stem cells *they produce calcifying cartilage matrix allow growth of the long bone *plate moves upward as a trail of newly formed bone spicules develop Periosteum vs Endosteum *most bones are covered by a layer of connective tissue (dense irregular) *well vascularised Periosteum *on the outer surface of bones *outer layer is fibrous *inner layer is osteogenic *is absent where tendons and ligaments insert in bones (enthesial bone) and at  articular surfaces (subchondral bone)   Endosteum *lines trabecular bone and the free surface of inner circumferential lamellae *marrow cavities contain reticular fibres and cells, blood vessels, as well as haemopoietic stem cells *but have no lymphatics  Osteoporosis Normal Bone Metabolism *requires regular turn over of all bone *rate and degree of turnover is determined by loading of all bone *i.e. lower limb bones have a higher turnover than upper limb bones Reduced Bone Metabolism *combination of loa dietary calcium and insufficient exercise *reduced ability to rebuild bone *reduced loading = thinner and fewer trabeculae = increased likelihood of disabling stress fracture with minimal loading  